A permanent magnet electric motor produces a rotational variation called a cogging torque. The cogging torque generates a vibration or noise to cause the control performance of electric motor to be degraded. Therefore, a method for reducing the cogging torque was disclosed in JP-A-4-21330. According to this patent, to remove two frequency components of the cogging torque, permanent magnets are arranged at four stages in the axial direction of a rotor iron core and shifted in the circumferential direction of the rotor iron core to give rise to a skew effect, so that a plurality of permanent magnets are dislocated in the circumferential direction depending on an axial position of the rotor, namely, arranged on the surface of the rotor iron core at a skew angle (hereinafter referred to a stage skew angle).
A theoretically calculated angle (hereinafter referred to as a theoretical angle θt) for the stage skew angle is employed. The theoretical angle θt1 for a fundamental wave component of the cogging torque is given byθt1=(360°/least common multiple of the number of stator magnetic poles and the number of rotor magnetic poles)/2  (3)
Further, the theoretical angle θt2 for the second order higher harmonic wave component of the cogging torque is half θt1.
However, if the theoretical angle of stage skew angle is actually applied to the permanent magnet electric motor, a reduced cogging torque is possibly less sufficient. Its reason is that an axial leakage flux occurs due to employment of the stage skew, but the influence of a magnetic saturation with the leakage flux is not considered. The leakage flux causing the cogging torque may occur at the stage portion of the permanent magnet and inside the rotor iron core, but the leakage flux inside the stator iron core is a main cause of the cogging torque. Therefore, there was a problem that the cogging torque was not fully reduced because the theoretical angle was employed for the skew angle of the rotor.
Also, the constitution of the rotor was complex to remove two frequency components of the cogging torque.